1. Field of the Invention
The present invention relates generally to switching circuits and voltage regulators and more particularly to a protection circuit for a tap switching line voltage regulator.
2. Background of the Invention
In the past thyristors have been used as the series switching element in tap switching regulators on each of the taps to be switched. The thyristor device in accordance with the appropriate control circuitry may be triggered "on" anytime the anode voltage is more positive than the cathode and will remain in the conducting state without a control signal being applied until the current through the device is reduced to less than a very small value referred to as the "holding current". Then the device will turn off until the voltage applied to the anode is positive with respect to the cathode and another trigger pulse is applied to the control gate.
The incoming power line sine-wave voltage reduces the current in the "on" device to zero once each half cycle. The control circuitry must apply a trigger pulse to the correct thyristor device once each half cycle so that the load has a continuous source of voltage.
If the load is resistive then the voltage and current will be in phase. The typical regulator control circuit will trigger the thyristors at either zero line voltage or zero line current which is suitable for resistive type loads. The real load will be either lagging power factor (inductive) or leading power factor (capacitive). The net result is either a switching transient the load, a loss of part of a half cycle of voltage, or a large circulating current between taps when two thyristor series switching devices are on at one time as with leading power factors; i.e. the next device is turned "on" while current is still flowing in the last device. The large circulating currents of the latter case may damage the thyristor switching devices while the first two cases may disturb the load.
Magnetic type components have been used in the past to force semiconductor devices to share load current when one device is not large enough to carry the total load current as shown in FIG. 4A. Multiple numbers of devices can be accommodated with increasing complexity, cost and the number of magnetic components as shown in FIGS. 4B and 4C. This arrangement works only for common input, common output load current sharing configurations; (reference General Electric SCR Manual Sixth Edition 176-177).